The present invention relates to an adjacent interference removal device for removing an influence of an interference signal caused by an adjacent interference, from a received signal received by an FM broadcasting receiver or the like.
The present application claims priority from Japanese Applications No. 2004-19710, the disclosures of which are incorporated herein by reference.
Generally, with regard to an FM broadcasting receiver or the like, if an interference signal from an adjacent broadcasting station has been generated adjacent to a received signal from a desired broadcasting station, a sort of reception quality deterioration will occur due to the adjacent interference.
Conventionally, when such an adjacent interference has occurred, an applicable method is to reduce, on the receiver side, the pass band width of an IF filter which passes an intermediate frequency signal (IF signal), thereby removing the influence of the interference signal (refer to Japanese Unexamined Patent Application Publication No. Hei 08-097738).
The receiver disclosed in Japanese Unexamined Patent Application Publication No. Hei 08-097738, as shown in FIG. 1 of the patent publication, comprises a first path having a first IF filter (9) and a third buffer circuit (16), a second path having a second IF filter (10) and a fourth buffer circuit (17), both paths being connected on the rear side of a mixer (2). The receiver further includes a first level detecting circuit (13), a second level detecting circuit (14), a comparison circuit (15), the third buffer circuit (16), and the fourth buffer circuit (17).
The first IF filter (9), as shown in FIG. 3 of the above-mentioned patent publication, has a pass band width of 150 KHz with a center frequency (carrier frequency) 10.7 MHz as its center, i.e., a pass band width of −75 KHz to +75 KHz with its center frequency being 0 KHz, thereby limiting the IF signal outputted from the mixer (2) within a band width of 150 KHz and then supplying the same to the third buffer circuit (16).
The second IF filter (10), as shown in FIG. 3 of the above-mentioned patent publication, has a pass bandwidth of 50 KHz with a center frequency as its center, i.e., a pass band width of −25 KHz to +25 KHz with its center frequency being 0 KHz, thereby limiting the IF signal outputted from the mixer (2) within a band width of 50 KHz and then supplying the same to the fourth buffer circuit (17).
The first level detecting circuit (13) detects the level of an IF signal (whose band has been limited within 150 KHz) outputted from the first IF filter (9). On the other hand, the second level detecting circuit (14) detects the level of an IF signal (whose band has been limited within 50 KHz) outputted from the second IF filter (10).
The comparison circuit (15) performs a comparison between the results detected by the first and second level detecting circuits (13) and (14). If the level of the IF signal outputted from the first IF filter (9) is higher than that of the IF signal outputted from the second IF filter (10), it can be determined that an interference signal is contained in the IF signal outputted from the first IF filter (9). Then, by switching the third buffer circuit (16) off, and the fourth buffer circuit (17) on, an IF signal outputted from the second IF filter (10) is transmitted to a detecting circuit (6) side through the aforementioned fourth buffer circuit (17). On the other hand, if the level of the IF signal outputted from the first IF filter (9) is equal to that of the IF signal outputted from the second IF filter (10), it can be determined that an interference signal is not existing. Then, by switching the third buffer circuit (16) on, and the fourth buffer circuit (17) off, the IF signal outputted from the second IF filter (10) is transmitted to the detecting circuit (6) side through the third buffer circuit (16).
In this way, the receiver disclosed in Japanese Unexamined Patent Application Publication No. Hei 08-097738, comprises the first IF filter (9) having a pass band width of 150 KHz, and the second IF filter (10) having a pass band width of 50 KHz. If an interference signal is contained in the IF signal outputted from the mixer (2), the receiver will operate to limit the band of the IF signal by virtue of the second IF filter (10) having a narrow pass band width.
However, although it is possible to remove the influence of an interference signal upon limiting the band of each IF signal within 50 KHz by virtue of the second IF filter (10), since a distortion occurs in a detection output signal outputted from the detecting circuit (6), it has been found difficult to improve an actual reception quality.
Moreover, for example in Europe (European nations), since channels of adjacent broadcasting stations are separated from each other only at 100 KHz, it is easy to receive an influence of an interference signal from an adjacent channel separated −50 KHz and +50 KHz from the center frequency of a desired signal. On the other hand, whenever receiving an influence of such an adjacent interference, the band of each IF signal will be reduced to within a narrow width of 50 KHz by virtue of the second IF filter (10), thereby making it possible to remove the influence of an interference signal. However, since a distortion occurs in a detection output signal outputted from the detecting circuit (6), it has been found difficult to improve an actual reception quality.